What Is Alt Text? The Ultimate Support Guide for Accessibility, SEO, and AI
A complete, practical guide to understanding and writing effective alternative text for your website images.
Editor’s Note: This article is an expanded English adaptation of the original piece published in the AEOvara blog. You can read the original Finnish version here: Mitä on ALT-teksti?
If you’ve ever wondered what goes on behind the scenes of a webpage, you’ve probably noticed that images aren’t just visual elements — they are also data. But how do search engines, screen readers, and AI tools understand an image if they can’t “see” it the way we do?
The answer is Alt Text (Alternative Text).
While it might seem like a minor technical detail, alt text is one of the most powerful tools you have to make your content accessible, searchable, and AI-friendly. Here’s everything you need to know.
What Exactly Is Alt Text?
Alt text is a brief, descriptive text embedded directly into an image’s HTML code using the alt attribute.
Example of alt text in code:
<img src="family-lakeside-sunset.jpg" alt="A family of four posing on the shores of Lake Saimaa during a golden sunset">
What it does: It tells screen readers, search engines, and AI systems what the image depicts and why it’s on the page. If an image fails to load, the browser will display this text in its place.
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What it is NOT:
- It is not a caption (which is visible to all users below the image).
- It is not the
titleattribute (which only shows a small tooltip when you hover over an image on desktop).
Alt text is the only one of these three that genuinely impacts both accessibility and search engine optimization.
Why Alt Text Matters: 3 Critical Reasons
1. Accessibility Comes First
Tens of thousands of people rely on screen readers daily to navigate the web. Without alt text, a screen reader will simply announce “image,” leaving the user in the dark. Providing a text alternative is not just a best practice; under the W3C’s WCAG guidelines (Success Criterion 1.1.1), it is a legal accessibility requirement for public and many private organizations worldwide.
2. Google Doesn’t “See” Images, It Reads Them
Google’s own documentation states that it uses alt text in conjunction with computer vision algorithms and surrounding page content to understand the image’s subject. While AI can guess that a photo contains “a person outdoors at sunset,” it doesn’t know it’s specifically “a family portrait session on Lake Saimaa” unless you provide that context. Good alt text directly improves your chances of ranking in Google Image Search.
3. AI and LLMs Read Alt Text (Not Pixels)
This is the most overlooked reason to care about alt text today. When AI tools like ChatGPT, Perplexity, or other web-browsing agents analyze a webpage, they do not “look” at images like humans do. They read the alt text as part of the page’s textual content. If your image lacks alt text, AI agents often dismiss it as purely decorative and ignore its context entirely when generating answers or summaries.
How to Write Great Alt Text: 5 Practical Rules
Writing effective alt text doesn’t have to be complicated. Follow these five guidelines:
- Describe the content, not just the format.
Focus on what is actually happening in the image and why it matters to the page. - Never start with “Image of” or “Photo of.”
Screen readers already announce the element as an image. Saying “Image of a dog” is redundant; just say “A golden retriever playing in the snow.” - Provide context, not just the subject.
A picture of a chart should be described differently depending on whether the article is about Q3 sales growth or general data visualization trends. - Avoid keyword stuffing.
Keep it natural. Search engines and AI penalize spammy, unnatural-sounding text. - Use empty alt text (
alt="") for decorative images.
If an image is purely for visual flair (like a decorative divider or background shape) and adds no informational value, use an empty alt attribute. This tells screen readers to safely skip it.
💡 Pro Tip: The “Phone Test”
If you’re ever stuck, use this simple mental exercise: Imagine calling a friend and describing the image to them over the phone. What you would naturally say to help them visualize it is usually the perfect alt text.
Aim for conciseness — typically between 80 to 125 characters is the sweet spot for clarity and compatibility.
The Bottom Line
Alt text is a small detail with a massive ripple effect. It determines whether a visually impaired user can fully experience your site, whether your visuals can be discovered via search engines, and whether modern AI tools recognize your content as valuable.
Take the extra few seconds to write it thoughtfully for every meaningful image. Your users, your SEO, and the AI of tomorrow will thank you.
About the Author
Jarno S. is the founder and lead strategist at AEOvara, a Finnish expertise agency specializing in Answer Engine Optimization (AEO), Generative Engine Optimization (GEO), and Large Language Model Optimization (LLMO).
Today, Jarno helps small and medium-sized businesses, growth companies, and expert brands secure their digital footprint. He ensures they are recognized as the primary, authoritative source by AI models like ChatGPT, Gemini, and Perplexity. Based in Lappeenranta, Finland, he works with clients nationwide, turning AI visibility into tangible business growth.
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